Feeding the Ocean? A biological mediation of estuarine silicon

  • Holly J Welsby

Student thesis: Doctoral ThesisDoctor of Philosophy (PhD)


Riverine dissolved silicon (DSi) and biogenic silica (BSi) are altered along the estuarine gradient. An important area controlling silicon (Si) transport in estuaries, which is often overlooked, is the benthic diatom-dominated biofilm system on intertidal mudflats. Here, the hypertidal Severn Estuary, UK has been used as a case study to improve our understanding of DSi and BSi transport in these benthic-dominated systems. Between 2016 and 2018, ecological surveys were carried out along the River Severn, and its estuary and tributaries (Wye, Usk, Avon, and Cardiff Bay), alongside surveys of the benthic biofilms on the intertidal mudflats. The first time-series datasets of DSi and BSi, as well as Si isotopes in the Severn are presented. Siliceous phytoplankton contributed to a small amount of the estuarine BSi budget, whilst riverine inputs of BSi (e.g. phytoliths), and benthic-sourced BSi, influenced the spatio-temporal changes in estuarine BSi. Relatively high BSi concentrations were measured in the upper Severn Estuary and were characterised by isotopically heavy waters. It was hypothesised that the BSi originated from the diatom-dominated biofilms on the intertidal mudflats. The biomass-rich biofilms had high productivity (chlorophyll fluorescence analysis), and a high potential to biologically mediate Si, contributing to isotopically heavy mudflat waters (+1.19-2.03‰). The fast-flowing tidal currents would have exceeded the erosion thresholds of the biofilms, despite the biofilms exhibiting sediment biostabilization properties (total and colloidal-S carbohydrates). These tidal currents likely eroded and transported the sediment-polymer-diatom matrix, and thus BSi into the water column (maximum of 91.4 mg/L), with SPM and BSi remaining tightly coupled in the near-bed suspension and the surf zone (bioflocculation). In agreement, a Silicon-Generic Estuarine Model (1D RTM) showed maximum erosion rates of sediment and BSi in the upper estuary. This study has improved our understanding of the complex processes governing Si transport in hypertidal, benthic-dominated estuaries.
Date of Award1 Oct 2019
Original languageEnglish
Awarding Institution
  • The University of Bristol
SupervisorMarian L Yallop (Supervisor) & Katharine Hendry (Supervisor)

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